Measuring the density, velocity and mass flow of gases

ABSTRACT

A meter and a method of measuring the velocity of gas in a duct are described, and gas density and mass flow may also be measured. In one form the meter has an ion source and two ion collectors, so that gas in the duct is first ionized and then passes the collectors. The source and the collectors are devised to present little or no impedance to gas flow. A voltage pulse is applied to the first collector and the interval between this pulse and the resulting effect in the number of ions collected at the second collector is measured to give gas velocity. Density measurement is by determining the number of ions collected between pulses at the second collector, and mass flow is obtained by forming the product of velocity and density.

United States Patent Brain [451 Aug. 29, 1972 Inventor: Thomas JohnStirrat Brain, Hamilton, Scotland Assignee: National ResearchDevelopment Corporation, London, England Filed: March 23, 1970 Appl.No.: 21,940

Foreign Application Priority Data March 28, 1969 Great Britain..l6,537/69 References Cited UNITED STATES PATENTS Stuart ..250/43.5 FCX Morgan ..250/43.5 FC X Campbell ..250/43.5 FC X 5/1953 Mellen..250/43.5 FC X OTHER PUBLICATIONS Peycelon et al., S.N. 376,930,Published by Alien Property Custodian, May 25, 1943.

Primary ExaminerArchie R. Borchelt Attorney-Cushman, Darby & Cushman[57] ABSTRACT A meter and a method of measuring the velocity of gas in aduct are described, and gas density and mass flow may also be measured.In one form the meter has an ion source and two ion collectors, so thatgas in the duct is first ionized and then passes the collectors. Thesource and the collectors are devised to present little or no impedanceto gas flow. A voltage pulse is applied to the first collector and theinterval between this pulse and the resulting effect in the number ofions collected at the second collector is measured to give gas velocity.Density measurement is by determining the number of ions collectedbetween pulses at the second collector, and mass flow is obtained byforming the product of velocity and density.

17 Claims, 2 Drawing Figures PULSE 1'1"1'121 osc. 4

I MULT. SMOOTH! 29 FLlP FLOP OCT -@2 MEASURING THE DENSITY, VELOCITY ANDMASS FLOW OF GASES The present invention relates to methods andapparatus for measuring the velocity of gases in a duct. In addition thedensity and/or mass flow of the gases may also be measured.

A known gas velocity meter employs an ionizing source between two spacedion collectors. The ratio of the ions collected by the collectors isthen an indication of the velocity of the gas. However this known meteris affected by variations in the ion recombination constant and is onlysuitable for the measurement of high gas velocities.

According to a first aspect of the present invention there is provided ameter for measuring the velocity of gases, including ionizing means forionizing at least a part of a gas flowing along a duct, first and secondelectrode means, for collecting ions, spaced apart in the duct, meansfor effecting a change in the number of ions collected by the firstelectrode means, and means for measuring the interval between the timeat which a change in the number of ions collected by the first electrodemeans is effected and the time at which a resultant change occurs in thenumber of ions collected at the second means, the meter being such thatthe velocity of gas flowing in the duct depends on the said interval.

According to a second aspect of the present invention there is provideda method of measuring the velocity of gases including ionizing at leasta part of a gas flowing along a duct, effecting a change in the numberof ions collected at a first point in the duct, collecting ions whichpass the first point at a second point in the duct and measuring theinterval between the time at which the change in the number of ionscollected at the first point is effected, and the time at which aresultant change occurs in the number of ions collected at the secondpoint.

The meter and method according to the invention do not suffer from theabove mentioned disadvantages of the known meter since transit time isnot affected by the ion recombination constant and relatively low aswell as high gas velocities can be measured.

In the meter according to the first aspect of the invention the ionizingmeans preferably includes the isotope Americium-24l (alph emitting) orthe isotope mixture Strontium90/Yt trium-90 (beta-emitting).

These isotopes may be placed in a sealed thin metal foil located aroundthe inner surface of the duct.

The first and second electrode means may each comprise two electrodes,the first electrode being a rod positioned downstream from theionization means, at the center of the duct, and the second electrodebeing an insulated metal section of the duct. The means for effecting achange in the number of ions collected by I depends on the gas velocity.The output circuit of the flip-flop may be connected to a volt meter byway of a smoothing circuit, so that the voltage indicated is a measureof gas velocity.

The meter for measuring gas velocity has many desirable features: sinceit presents little or no obstruction to gas flow, there will be only anegligible head loss across it. The meter is easy to install and nosealing problems are encountered. Since there are no moving parts themeter will not wear out, and there is no need to convert a mechanicalsignal to an electrical signal. Calibration of the meter is linear.

The meter according to the invention may also be used to measure thedensity of gas flowing in the duct by the addition of bias means forapplying a unidirectional voltage between the first and secondelectrodes of one of the electrode means, and means for providing asignal indicative of the number of ions collected by the said oneelectrode means in the absence of a voltage from the pulse source, forexample between pulses. The means for indicating the number of ionscollected may be an ammeter connected in series with a direct-currentsource.

A versatile mass-flow meter can be provided by adding means forproviding a signal indicative of the number of ions collected by thesecond means, and means for multiplying the said signal by the output ofthe means for measuring the interval. For example, the voltage obtainedfrom the above mentioned smoothing circuit may be multiplied by thecurrent flowing through the above mentioned d.c. source using a Wattmeter or an equivalent circuit. The pulse rate of the pulse sourceshould be such that the number of ions collected at the second means inbetween pulses correctly indicates the density of the gas flowing in thepipe.

The above specified mass flow meter has the advantages already listedfor the velocity meter but in addition flow rate integration can readilybe achieved to give the total gas flow. The flow meter will give partresponse to changes in gas flow, and it can, of course, be used as avelocity meter or a density meter.

Certain embodiments of the invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a part-schematic part-block diagram of one embodiment of amass flow-meter according to the invention, and

FIG. 2 is a schematic drawing of an electrode arrangement which may beused instead of the arrangement shown in FIG. 1.

In FIG. 1 gas passes in the direction of an arrow 10 along a pipe 11.The gas is first ionized by radioactive powder 12 sealed within a thinmetal foil 13 located around the inner circumference of the pipe. As hasbeen mentioned, the powder may contain the radioactive isotopeAmericium-24l (alpha emitting) or the isotope mixtureStrontium-/Yttrium-9O (beta emitting).

After being ionized the gas passes to an ion collection section 14 ofthe pipe 11 comprising two electrodes; firstly a cylindrical rodelectrode 15 located at the center of the pipe co-axial therewith, andsecondly an electrode 16 which is a section of the pipe wall insulatedfrom the remainder of the pipe by insulating sections 17 and 18.

Gas then passes to a second ion-collection section 19 of the same formas the first such section. This second section has a cylindrical centerelectrode 20 and a wallsection electrode 21 insulated from the remainderof the pipe.

The electrodes 15 and 20 are supported by brass rods (not shown) within,and radial to, the pipe 11. The rods are fixed to, but insulated from,the electrodes 16 and 21, and provide a means for making connections tothe electrodes 15 and 20.

A square-wave voltage from a pulse oscillator 22 having a repetitionfrequency of approximately 100 c/s is applied between the electrodes 15and 116. The effect is that for the duration of each voltage pulse anumber of ions are collected in the section 14, and in the intervalsbetween pulses when zero voltage is applied between the electrodeslS and16, no ions are collected in this section. The interval between theleading edge of pulses from the oscillator 22 and the corresponding fallin the number of ions collected at the electrode 20 depends on thevelocity of the gas in the pipe. Each time theleading edge of a pulsefrom the oscillator occurs, a flip-flop circuit 23 is set to aparticular one of. its two states, the flip-flop circuit being reset bya signal from a detector circuit 24 when a decrease in the number ofions collected at the electrode 20 is detected. Thus the interval forwhich the flip-flop circuit 23 is in its said one state depends on thevelocity of the gas. The pulse output from the flip-flop circuit 23 ispassed to a smoothing circuit 25 whose output is connected to a dc. voltmeter 26 which indicates the velocity of gas flow directly.

The electrode 20 is connected in series with a direct current meter 27and a battery 28 which applies a unidirectional voltage of about 120volts between the electrodes 20 and 21.

The detector 24 may include a resistor (not shown) in series with thebattery 28, and a trigger circuit (not shown) responsive to changes involtage across the resistor. Hence when the current in the resistor dueto the collection of ions changes, the trigger circuit changes the stateof the flip-flop circuit 23.

The number of ions arriving at the electrode 20 is found by measuringthe battery current and is dependent on the total strength of the sourceand the ionization cross-section of the gas which are fixed for a givenmeter and gas. The number is also generally dependent on the gasvelocity and density at the ionizing section but for a particular sourcestrength, over a given velocity range, the ion collection current, asindicated by the meter 27, is independent of velocity and for a gas ofconstant composition it may be shown that in the intervals when no ionsare collected at section 14 and all the ions arriving at section 19 arecollected, the ion collection current,

where A cross-sectional area of the pipe J ionizing power of the sourcef the ionization cross section of the gas p the density of the gas ationizing point p the density of the gas at S.T.P. I the collectorcurrent e the charge on an electron therefore I Kp where K is aconstant.

The ionizing power, J, is a factor depending on the source strength. Theionization cross-section of the gas, f, gives a measure of the abilityof the gas to be ionized. The product Jf is the number of ion pairs perunit volume of gas produced when the gas is moving at unit velocity atS.T.P. conditions. Thus ionization current as indicated by the meter 27gives a measure of the gas density (p) in the pipe adjacent to theionizing source.

For accurate measurement of gas density, the composition of the gasshould be constant, dust content should be low, and temperaturevariations between the sections 14 and 19 should be small.

The accuracy of density measurement required, determines the velocityrange in which density measurement can be carried out. For example theerror in density measurement will be e 0.01 per cent if where o'=p/p )t=the ion recombination coefficient, 7

p. the velocity of the gas at the ionizing section, and

L the distance between the ionizing source and the second ion-collectionsection.

The voltage from the smoothing circuit 25 and the current in the meter27 are applied to a circuit 29 which multiplies the average value of thecurrent by the value of the voltage to give a product which isproportional to the mass flow of gas in the pipe. The circuit 29 may bea Watt meter. The current flowing in the meter 27 will be a pulsecurrent, but provided the response time of this meter and that of thecircuit 29 is low compared with the frequency of the pulse oscillator22, steady accurate output readings will be obtained.

The electrodes of FIG. 1 may be replaced by the electrode arrangement ofF IG. 2 which has the advantage that there is no obstruction to gasflow.

The electrodes 15 and 16 are replaced by sheet metal electrodes 31 and32 respectively. Similar but rather larger sheet metal electrodes 33 and34 replace the electrodes 20 and 21. The radioactive source 12 isconstructed as in FIG. 1, and the electrodes and the source arecontained in an insulating tube 35 which lines the pipe 11. Theelectrodes 31 and 32 are connected to the live and the earth side of thepulse oscillator 22, respectively, the electrode 33 is connected to thehigh voltage terminal of the battery 28, and the electrode 34 isconnected to the direct current meter 27.

The configuration of the ion source and the electrodes need not be asdescribed, and other circuits from that indicated by the block diagrammay be used to evaluate density, velocity and mass flow.

I claim:

1. A meter for measuring the velocity of gases, including:

ionizing means for ionizing at least a part of a gas flowing along aduct,

first and second electrode means, for collecting ions,

spaced apart in the duct,

means for effecting a change in the number of ions collected by thefirst electrode means, and

means for measuring the interval between the time at which a change inthe number of ions collected by the first electrode means is effectedand the time at which a resultant change occurs in the number of ionscollected at the second electrode means.

2. A meter according to claim 1 wherein the means for ionizing gasincludes a radioactive source.

3. A meter according to claim 2 wherein the radioactive source includesan isotope chosen from the group including the isotope Americium-24l andthe isotope mixture Strontium-90/Yttrium-90.

4. A meter according to claim 2 wherein the first and second electrodemeans each include a first electrode in the form of a rod positioned atthe center of the duct, and a second electrode in the form of aninsulated conductive portion of the duct, the rod being surrounded by,and co-axial with, the portion.

5. A meter according to claim 4 wherein the means for effecting a changein the number of ions collected by the first electrode means includes apulse source adapted to supply a square wave voltage to the firstelectrode means.

6. A meter according to claim 5 including:

a direct current source for applying a bias voltage to one of theelectrode means, and

means for providing a signal indicative of the number of ions collectedby the electrode means to which the bias voltage is applied when thepulse source does not apply a voltage to the first electrode means, themeter being such that the said signal may indicate the density of gas inthe duct.

7. A meter according to claim 6 including means for multiplying theoutput signal from the means for measuring the said interval by thesignal indicative of the number of ions collected.

8. A meter for measuring the velocity of gases, including:

ionizing means for ionizing at least a part of a gas flowing along aduct,

first and second electrode means, for collecting ions,

spaced apart in the duct,

means for effecting a change in the number of ions collected by thefirst electrode means, and

means for measuring the interval between the time at which a change inthe number of ions collected by the first electrode means is effectedand the time at which a resultant change occurs in the number of ionscollected at the second means, wherein said means for ionizing gasincluding a radioactive source, and

the first and second electrode means each include first and secondelectrodes each in the form of a sheet of conductive material adjacentto, but separated from, a portion of the duct wall, each sheet beingshaped to conform to that portion of the wall to which it is adjacent.

9. A meter according to claim 8 wherein each second electrode has anaperture containing the first electrode.

10. A meter according to claim 8 wherein the means for effecting achange in the number of ions collected by the first electrode meansincludes a pulse source adapted to supply a square wave voltage to thefirst electrode means.

11. A meter according to claim 10 wherein the means for measuring theinterval includes;

detector means for detecting changes in the number of ions collected bythe second electrode means, and

a bistable circuit connected to means for determining the ratio of theintervals in which the bistable circuit is in first and secondconduction states,

the bistable circuit being connected to be triggered by the pulse sourceto take up a first conduction state when a voltage pulse is applied tothe first electrode means and to be triggered by a detection means totake up a second conduction state when a change in the number of ionscollected by the second electrode means is detected by the detectionmeans.

12. A meter according to claim 11 wherein the means for determining theratio of the intervals includes a volt meter connected to the bistablecircuit by way of a smoothing circuit.

13. A meter according to claim 10 including:

a direct current source for applying a bias voltage to one of theelectrode means, and

means for providing a signal indicative of the number of ions collectedby the electrode means to which the bias voltage is applied when thepulse source does not apply a voltage to the first electrode means, themeter being such that the said signal may indicate the density of gas inthe duct.

14. A meter according to claim 10 including:

a direct current source for applying a bias voltage to one of theelectrode means,

means for providing a signal indicative of the number of ions collectedby the electrode means to which the bias voltage is applied when thepulse source does not apply a voltage to the first electrode means, themeter being such that the said signal may indicate the density of gas inthe duct, and wherein the direct-current source is connected between thesaid first electrode and the said second electrode of that electrodemeans to which the bias voltage is to be applied.

15. A meter according to claim 10 including:

a direct current source for applying a bias voltage to one of theelectrode means,

means for providing a signal indicative of the number of ions collectedby the electrode means to which the bias voltage is applied when thepulse source does not apply a voltage to the first electrode means, themeter being such that the said signal may indicate the density of gas inthe duct, and means or multiplying the output signal from the means formeasuring the said interval by the signal indicative of the number ofions collected.

16. A method of measuring the velocity of gases including:

ionizing at least a part of a gas flowing along a duct,

effecting a change in the number of ions collected at a first point inthe duct,

collecting ions which pass the first point at a second point in theduct, and

measuring the interval between the time at which the change in thenumber of ions collected at the first point is effected, and the time atwhich a resultant change occurs in the number of ions collected at thesecond point.

17. A meter for measuring the velocity of gases, including:

ionizing means for ionizing at least a part of a gas flowing along aduct,

first and second electrode means, for collecting ions,

spaced apart in the duct,

a pulse source means for supplying a square wave voltage to the firstelectrode means, and

means for measuring the time interval between the time at which a changein the number of ions collected by the first electrode means occurs dueto the square wave voltage and the time at which a resultant changeoccurs in the number of ions collected at the second electrode means.

1. A meter for measuring the velocity of gases, including: ionizingmeans for ionizing at least a part of a gas flowing along a duct, firstand second electrode means, for collecting ions, spaced apart in theduct, means for effecting a change in the number of ions collected bythe first electrode means, and means for measuring the interval betweenthe time at which a change in the number of ions collected by the firstelectrode means is effected and the time at which a resultant changeoccurs in the number of ions collected at the second electrode means. 2.A meter according to claim 1 wherein the means for ionizing gas includesa radioactive source.
 3. A meter according to claim 2 wherein theradioactive source includes an isotope chosen from the group includingthe isotope Americium-241 and the isotope mixtureStrontium-90/Yttrium-90.
 4. A meter according to claim 2 wherein thefirst and second electrode means each include a first electrode in theform of a rod positioned at the center of the duct, and a secondelectrode in the form of an insulated conductive portion of the duct,the rod being surrounded by, and co-axial with, the portion.
 5. A meteraccording to claim 4 wherein the means for effecting a change in thenumber of ions collected by the first electrode means includes a pulsesource adapted to supply a square wave voltage to the first electrodemeans.
 6. A meter according to claim 5 including: a direct currentsource for applying a bias voltage to one of the electrode means, andmeans for providing a signal indicative of the number of ions collectedby the electrode means to which the bias voltage is applied when thepulse source does not apply a voltage to the first electrode means, themeter being such that the said signal may indicate the density of gas inthe duct.
 7. A meter according to claim 6 including means formultiplying the output signal from the means for measuring the saidinterval by the signal indicative of the number of ions collected.
 8. Ameter for measuring the velocity of gases, including: ionizing means forionizing at least a part of a gas flowing along a duct, first and secondelectrode means, for collecting ions, spaced apart in the duct, meansfor effecting a change in the number of ions collected by the firstelectrode means, and means for measuring the interval between the timeat which a change in the number of ions collected by the first electrodemeans is effected and the time at which a resultant change occurs in thenumber of ions collected at the second means, wherein said means forionizing gas including a radioactive source, and the first and secondelectrode means each include first and second electrodes each in theform of a sheet of conductive material adjacent to, but separated from,a portion of the duct wall, each sheet being shaped to conform to thatportion of the wall to which it is adjacent.
 9. A meter according toclaim 8 wherein each second electrode has an aperture containing thefirst electrode.
 10. A meter according to claim 8 wherein the means foreffecting a change in the number of ions collected by the firstelectrode means includes a pulse source adapted to supply a square wavevoltage to the first electrode means.
 11. A meter according to claim 10wherein the means for measuring the interval includes; detector meansfor detecting changes in the number of ions collected by the secondelectrode means, and a bistable circuit connected to means fordetermining the ratio of the intervals in which the bistable circuit isin first and second conduction states, the bistable circuit beingconnected to be triggered by the pulse source to take up a firstconduction state when a voltage pulse is applied to the first electrodemeans and to be triggered by a detection means to take up a secondconduction state when a change in the number of ions collected by thesecond electrode means is detected by the detection means.
 12. A meteraccording to claim 11 wherein the means for determining the ratio of theintervals includes a volt meter connected to the bistable circuit by wayof a smoothing circuit.
 13. A meter according to claim 10 including: adirect current source for applying a bias voltage to one of theelectrode means, and means for providing a signal indicative of thenumber of ions collected by the electrode means to which the biasvoltage is applied when the pulse source does not apply a voltage to thefirst electrode means, the meter being such that the said signal mayindicate the density of gas in the duct.
 14. A meter according to claim10 including: a direct current source for applying a bias voltage to oneof the electrode means, means for providing a signal indicative of thenumber of ions collected by the electrode means to which the biasvoltage is applied when the pulse source does not apply a voltage to thefirst electrode means, the meter being such that the said signal mayindicate the density of gas in the duct, and wherein the direct-currentsource is connected between the said first electrode and the said secondelectrode of that electrode means to which the bias voltage is to beapplied.
 15. A meter according to claim 10 including: a direct currentsource for applying a bias voltage to one of the electrode means, meansfor providing a signal indicative of the number of ions collected by theelectrode means to which the bias voltage is applied when the pulsesource does not apply a voltage to the first electrode means, the meterbeing such that the said signal may indicate the desnity of gas in theduct, and means or multiplying the output signal from the means formeasuring the said interval by the signal indicative of the number ofions collected.
 16. A method of measuring the velocity of gasesincluding: ionizing at least a part of a gas floWing along a duct,effecting a change in the number of ions collected at a first point inthe duct, collecting ions which pass the first point at a second pointin the duct, and measuring the interval between the time at which thechange in the number of ions collected at the first point is effected,and the time at which a resultant change occurs in the number of ionscollected at the second point.
 17. A meter for measuring the velocity ofgases, including: ionizing means for ionizing at least a part of a gasflowing along a duct, first and second electrode means, for collectingions, spaced apart in the duct, a pulse source means for supplying asquare wave voltage to the first electrode means, and means formeasuring the time interval between the time at which a change in thenumber of ions collected by the first electrode means occurs due to thesquare wave voltage and the time at which a resultant change occurs inthe number of ions collected at the second electrode means.